1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to neutron porosity measurement devices for oil and gas industry, more particularly, devices including plural semiconductor neutron detection cells arranged to allow gathering data with azimuth coverage and enabling evaluating porosity by considering different configurations.
2. Discussion of the Background
In the oil and gas industry, well logging (or borehole logging) is a term used for detail records made about geophysical properties of geological formations penetrated by a borehole. The records include results of various and sometimes complex measurements performed using tools lowered into the well or samples brought to the surface. Of particular interest are measurements of porosity, permeability and fluid content of the formations.
Porosity is the proportion of fluid-filled space found within the rock that may contain oil and gas, and is an indicator of the possible reserve of oil and gas. Logging tools configured to provide porosity measurements may employ various techniques (e.g., accoustics and Nuclear Magnetic Resonance), but using neutrons is the most frequently used method. The logging tools may have to operate at temperatures as high as 175° C. and sometimes even higher.
FIG. 1 is an axial cross section of a conventional down-hole porosity measurement set-up performed using a neutron source 10 and two detectors, a “near” neutron detector 20 and a “far” neutron detector 30, which are located at different distances from the neutron source 10. The neutron source 10 and the neutron detectors 20 and 30 are usually encapsulated in a chassis 40. The chassis 40 is lowered in a borehole 50 that penetrates a geological formation 60. Some of the neutrons emitted by the neutron source 10 towards the formation 60, loose energy (i.e., are “thermalized”) and are deflected towards the neutron detectors 20 and 30 due to collisions or interactions with nuclei in the formation 60.
The detectors 20 and 30 detect some (depending on each detector's efficiency) of the neutrons with lower (thermal) energy deflected towards the detectors. A ratio of the counting rates (i.e., number of detected neutrons/time) in the two detectors 20 and 30 is directly related to the porosity of the formation 60.
The probability of an interaction of a neutron and a nucleus (i.e., a nuclear reaction) can be described by a cross-section of the interaction (i.e., reaction). A detector's efficiency is proportional with the probability of an interaction occurring when a neutron enters the detector's volume. The neutron detectors are built based on the large probability (i.e., cross-section) of a thermal neutron being captured (i.e., interact/react) with one of the three nuclei: helium (3He), lithium (6Li) or boron (10B). Other particles such as, the α particle (24α) and the proton (11p) result from the reaction of the thermal neutron with these elements. A calculable amount of energy (Q) is emitted as a result of the neutron capture reaction. This emitted energy may be kinetic energy of the resulting particles or gamma rays. The energy is dissipated by ionization, that is, formation of pairs of electron and positively charged particle. These pairs can be collected, for example, in an electrical field, and, thus, generate a signal recognizable as a signature of the neutron capture reaction. The larger is the emitted energy, the larger is the amplitude of the signature signal.
Some other particles besides the targeted neutrons (e.g., gamma rays) may cross the detector and be detected simultaneously. A good detector should exhibit characteristics that would allow discrimination between capture of a thermal neutron and other untargeted nuclear reactions that may occur. To facilitate discrimination between a neutron capture reaction and a gamma ray, the energy emitted in the neutron capture reaction (Q) should be as high as possible.
The three most common neutron capture reactions used for neutron detection are illustrated in Table 1:
TABLE 1Thermal neutron cross NameReactionQ (MeV)section (barns)10B(n, α)510B + 01n → 37Li + 24αGround 2.7923840Excited 2.316Li(n, α)36Li + 01n → 13H + 24α4.789403He(n, p)23He + 01n → 13H + 11p 0.7645330
In the above table, relative to the 10B(n, α) reaction “Ground” means that the resulting 37Li is in a ground state and “Excited” means that the resulting 37Li is in the first excited state.
Traditionally, detectors based on 3He(n, p) reaction have been used in neutron porosity measurements performed in the oil and gas industry, due to their relatively low cost, ruggedness, good detection efficiency, and insensitivity to gamma rays (i.e., the cross section for an interaction of the gamma ray with 3He is very small). The detection efficiency of these 3He based detectors can be improved by using higher pressures of the 3He gas, but the use of higher pressures results in increasing the cost of the detectors and of the high voltage required to operate them, which adversely affects the associated detector electronics. Additionally, the critical worldwide shortage of 3He makes it necessary to develop alternate neutron detectors for neutron porosity measurements in the oil and gas industry.
Lithium-glass scintillation detectors are currently used in some logging tools. The detection efficiency of the detectors based on 6Li(n, α) reaction depends on the amount of 6Li in the detector material. A common lithium-glass used for down-hole logging is GS20, which has an isotopic ratio of 95% 6Li and a total lithium composition of 6.6%. Although the cross section for an interaction of the gamma ray with 6Li is significant, the large amount of energy (Q) resulting from the 6Li(n, α) reaction enables a reasonable discrimination from reactions induced by gamma rays. However, the poor energy resolution of lithium-glass detectors at room temperature diminishes further at temperatures as low as 150° C., rendering their use limited to relatively shallow wells. In the lithium-glass scintillation detectors, the lithium-glass is coupled to a photomultiplier tube (PMT) that introduces electronic noise at elevated temperatures and is mechanically fragile.
Downhole neutron-porosity measurements may be performed during or after the drilling of a well. Accordingly, tools come in two different conveyances, logging (or measuring) while drilling (LWD/MWD) and wireline. The principal difference between LWD and wireline systems is the service environment. LWD tools operate during the drilling process and are subjected to the high levels of vibration and shock generated by drilling through rock. Wireline tools are conveyed in and out of the borehole on a cable after drilling and do not experience the shock and vibration seen during drilling.
FIG. 2 is a transversal cross-section (which is perpendicular to the well axis) of a porosity measurement set-up using a conventional LWD tool 80. Inside the geological formation 82, a borehole 84 is drilled by a drill bit (not shown). During drilling, mud is circulated inside the well, to maintain a hydrostatic pressure to counter-balance the pressure of fluids coming out of the well, and to cool the drill bit while also carrying crushed or cut rock at the surface though the borehole 84. The tool 80 is configured such as not to interfere with the mud circulation, for example, by surrounding a mud channel 86. The clean mud is sent downhole through the mud channel 86 and carries the drilling debris up to the surface through the borehole 84.
The diameter of the tool 80 may be about 8 inch. Similar to FIG. 1, the tool 80 may include a “near” detector positioned at about 10 inches from a neutron source (not shown in FIG. 2, e.g. 10 in FIG. 1) and a “far” detector positioned at about 20 inches from the neutron source. The near and far detectors include one or more 3He tubes having a ¾ to 1 inch or more diameter and 2-4 inches length.
FIG. 2 is a transversal cross-section (perpendicular to the well direction) of a porosity measurement setup using a conventional tool, the cross-section cutting though either the “near” detectors or though the “far” detectors. In this case, the (near or far) detectors includes four 3He tubes 90. The 3He tubes 90 may be placed on a side closest to the formation 82, since capturing neutrons scattered from the formation 82 are of interest (rather than neutrons scattered by the mud flowing in the borehole 84 or the mud channel 86). The counting rates in the near and far detector may be corrected for the effect of neutrons scattered by the mud flowing in the borehole 84 or the mud channel 86. However, even when corrections are performed, asymmetrical measurements unavoidably introduce uncertainty and errors.
Accordingly, it would be desirable to provide neutron detectors having a good detection efficiency (i.e., large cross section for neutron capture), good discrimination relative to gamma rays, and can be used in the logging shock and vibration environment (e.g., during drilling) and at high temperatures (e.g., over 175° C.) and have a complete azimuth coverage that would enable taking into consideration more accurately the effect of neutrons scattered by the mud flowing in the borehole.